Network port - number identified system resources allocated from the application executable on some network host, for communicating with applications performed on other network hosts (including other applications on the same host).
For each of TCP and UDP standard defines simultaneous allocation to host up to 65,536 unique ports, identified by numbers from 0 to 65535. When sending network port number in the packet header is used (along with the IP-address of the host) to address specific application (and concrete, proper to him, network connection).
In a typical client-server model application or waiting for input (or connection request, "Listening Port", the role of the server) or sends data (or connection request) to the famous port, open application server (client role).
Default application seems random port (for example, near a free, large 1023) number. If necessary, the application can request specific (predetermined) port number. For example, web servers usually wait for open connection is defined protocol port 80 TCP.
Ports TCP ports not intersect with UDP. That is, the TCP port 7654 will not prevent exchange for UDP on port 7654.
In most UNIX-like operating systems listening on port numbers 0-1023 (almost all of whom are registered) requires special privileges. Each of the other ports may be busy asking him first process. However, the registered number is much higher than 1024.
Several standardized port numbers.
List of supported non-profit organization IANA.